1. Field of the Invention
This invention relates to an impulse-reducing X-ray debris shield useful in X-ray microlithography.
2. Description of the Prior Art
Microelectronic devices such as semiconductor chips are formed through the use of microlithography. In general, microlithography comprises imagewise exposing a photosensitive resin supported on a substrate to an imaging wavelength(s) of electromagnetic radiation. The imaging radiation can be uv, X-ray, etc. The radiation is applied imagewise by projecting the radiation through a mask. The design on the mask selectively blocks out the radiation and thereby forms a corresponding image in the photosensitive resin. This image, either positive or negative, is subsequently developed by applying a solvent in order to selectively remove the unexposed areas (a negative resist) or the exposed areas (a positive resist). The result is the formation of a planer image. The developed photoresist and substrate are subsequently treated by etching, stripping, etc. in order to form the final microelectronic device.
X-ray microlithography is based on the use of X-rays as the imaging irradiation. Because X-ray sources, which are typically very hot plasmas, expel hot gases, micron-sized particles, charged particles, and other debris, it is conventional to employ a debris shield or X-ray window between the X-ray source and the mask. Ideally, the debris shield should not absorb or attenuate any of the X-ray irradiation while stopping all debris. However, in practice, the materials used for such X-ray transmissive debris shields do attenuate the X-ray radiation to some extent. The attenuation of the X-ray radiation reduces the energy of the exposure and thus causes an increase in the exposure time of the resist.
To avoid this attenuation of the X-ray radiation, the debris shields have been made very thin, such as around 1-25 microns in thickness. Because there is less material for the X-rays to pass through, a thinner debris shield will attenuate less X-ray radiation than a thicker debris shield. However, a thinner debris shield is structurally less durable than a thicker debris shield. Accordingly, thinner debris shields break more easily and often in response to the forces exerted thereon by the X-ray source. Indeed, today a conventional thin X-ray debris shield will typically last for two to six exposures before breaking. Once the debris shield breaks, the photoresist that was being exposed during the break is discarded and a new debris shield and fresh photoresist are placed into the exposure device.
Accordingly, the prior art faced a dilemma. On the one hand, thicker debris shields could be used in order to have less down time and less product waste. However, such a solution came at the cost of longer exposure times and thus a slower output. On the other hand, thin debris shields could be employed which would absorb only small amounts of X-ray radiation and thereby allow quicker exposure times and higher production speeds. But this solution suffered from frequent downtime due to the rapid breakage of the thin debris shield. A technique for providing both low attenuation of X-ray radiation so that short exposure times can be maintained and structural durability so that the debris shield will have a longer life and require less replacement is thus needed.